In one conventional example of power conversion apparatuses that perform full-wave rectification for a three-phase AC power supply to output DC power, six rectification devices such as diodes are connected in series two by two, the anodes thereof are connected to each other, and the cathodes thereof are connected to each other, to form a bridge circuit which is used as a full-wave rectification circuit. A smoothing capacitor is provided on an output side of the full-wave rectification circuit. The respective phases of the three-phase AC power supply are supplied to the middle points of the above series connections.
However, in such a power conversion apparatus, since pulse current with a high peak flows on an output side of the three-phase AC power supply, power supply current includes many distortion components, that is, harmonic components, whereby the supply power factor decreases to 0.6 to 0.7. The decrease in the supply power factor results in increase in the size and the cost of the power conversion apparatus.
Accordingly, in order to improve the supply power factor of a three-phase AC power supply, a conventional technique disclosed in Patent Document 1 shown below proposes the following configuration. That is, AC reactors for the respective phases are provided between an AC power supply and a full-wave rectification circuit, and a converter is connected to the output side of the full-wave rectification circuit. Conduction intervals of the respective phases of current on the AC power supply side are prolonged by ON/OFF operation of a switching device of the converter and by the AC reactors provided on the AC side of the full-wave rectification circuit, and variation in the AC current is smoothed by the AC reactors on the power supply side. Thus, distortion components of current and voltage on the AC power supply side are decreased and the supply power factor is improved.
In addition, a conventional technique disclosed in Patent Document 2 shown below proposes the following configuration. That is, a single-phase rectification circuit and a converter circuit for improving power factor are connected to each phase output of the three phases, whereby each phase voltage of the three phases is individually rectified to make an input current waveform close to a sine wave, and power is supplied to a load with the supply power factor being increased to be close to 1 by the converter circuit for improving power factor.
Further, a conventional technique disclosed in Patent Document 3 shown below proposes the following configuration. That is, a step-up converter which steps up an output voltage of a three-phase rectification circuit is provided, the voltage of a smoothing device connected to the output of the step-up converter and the output current of the three-phase rectification circuit are detected, a DC current instruction for suppressing the deviation between a voltage instruction and the detected voltage value to zero is generated by a DC current instruction generation circuit, a pulse signal for suppressing the deviation between the DC current instruction and the detected current value to zero is generated by a pulse signal generation circuit, and the output current of the rectification circuit is converted to DC current through ON/OFF control for a switching device of the step-up converter by using the pulse signal, thereby enabling square wave current to flow for each phase in intervals of 120 degrees around the maximum and the minimum of instantaneous phase voltage of a three-phase power supply, and thus improving the supply power factor.